Liquid crystal displays are widely employed as a display of liquid crystal TVs and computers since they can be directly connected to IC circuits and particularly, their thickness can be decreased. Such liquid crystal displays are basically structured in such a manner that polarizing plates are arranged on both sides of a liquid crystal cell.
In such liquid crystal displays, in view of contrast, a liquid crystal display employing twisted magnetic (TN) at a twist angle of 90 degrees, and a liquid crystal display employing super-twisted magnetic (STN) at a twist angle of at least 160 degrees have been developed. Recently, however, a vertical alignment (hereinafter also referred to as VA) type liquid crystal display has been developed. The VA type liquid crystal display exhibits features in which black reproduction is improved employing a liquid crystal cell of a so-called vertical orientation mode, contrast is higher, and the viewing angle is relatively wide compared to that of the TN and STN types.
However, as a liquid crystal image plane increases as seen in a large type TV, demand for an increase in the viewing angle has been pronounced, and to increase the viewing angle, retardation films have been employed. Consequently, the width of the retardation film tends to be more widened due to increased liquid crystal image planes.
For the above purpose, application of polymer films has been investigated. A TAC film commonly exhibits a constant retardation value (Rt) in the thickness direction, but results in a very small retardation value (Ro) in the in-plane direction. As a result, the TAC film has not been suitable to improve the viewing angle of displays such as the above VA type LCD.
Known as a film to overcome the above drawbacks is one exhibiting excellent uniformity of the retardation value, which is employed as a retardation film exhibiting a slow axis in the lateral direction, as well as a polarizing plate protecting film, by stretching a cellulose ester film in the lateral direction during casting.
However, in a large liquid crystal display, it is common knowledge that in the employed retardation film, the shift of the slow axis in the film plane results in a significant decrease in contrast. In order to secure the exhibited performance, a technique is disclosed which pays attention to the shift of the slow axis (refer, for example, to Patent Documents 1 and 2).
Furthermore disclosed as a specific method to retard the shift of the slow axis of a retardation film is one in which, by regulating the temperature during stretching to soften a film, non-uniform stretching is minimized (refer, for example, to Patent Document 3).
Further disclosed as a tenter stretching method is a technique in which a web is stretched in the first tenter device, and subsequently, in the second tenter device, the resulting web is further stretched in the lateral direction upon maintaining the web width at a constant value (refer, for example, to Patent Document 4).
Still further disclosed as a tenter stretching method is the following one (refer, for example, Patent Document 5). By employing a method to produce a stretched film via the first process in which a film is pre-heated at a constant distance between the clips, the second process in which the film is stretched in the lateral direction while gradually increasing the distance between the clips, and the third process in which the film is stretched in the lateral direction while gradually further increasing the distance between the clips, it is possible to produce a stretched film, composed of thermoplastic norbornene based resins, which results in no formation of bowing, exhibits uniform physical properties in the lateral direction and results in uniform thickness and uniform retardation.
However, as a retardation film for large displays, at present, any of these results in problems of contrast, dimensional stability against humidity and heat, and light leakage, and thus improvements of them have been sought.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (herein after referred to as JP-A) No. 11-160536
(Patent Document 2) JP-A No. 2002-22943
(Patent Document 3) JP-A No. 2001-215332
(Patent Document 4) JP-A No. 2002-311245
(Patent Document 5) JP-A No. 2005-254812